Recently, in order to improve the performance and precision of wire cut spark erosion equipment, the wire-electrode to be used for wire-cut spark erosion has a high-tensile strength, e.g. a brass (Copper-Zinc alloy). Therefore, when one tries to cut the wire in a conventional manner by strongly pulling the wire-electrode by gripping it by hand or grasping it with the hand with a portion thereof wound around a finger, a large force is required to cut the wire and there is also the danger of cutting the hand and finger of the operation.
As a consequence, it can be considered to use pliers, cutting nippers, cutting pliers, or other kinds of metal scissors to cut the wire electrode. However, cutting by means of such tools causes various types of burrs, bending portions, damaged portion, etc. at the cutting edge portions by the wire-electrode. In many cases, the diameter of the tool's cutting surface is substantially the same as that of the wire-electrode used, or the cutting surface is pressed flat with a large maximum diameter. Thus, there is the problem in the subsequent wire-cut spark erosion that it is difficult to smoothyl perform the installing work of the wire-electrode by passing it through a pair of upper and lower wire-electrode guides, working fluid injecting nozzles, work starting holes of the workpiece, or the like.
Because of such reasons, upon the cutting of a wire-electrode, the cutting portion is pulled and stretched so as to be thin by applying a tensile force to the wire electrode to cut the wire while causing work hardening, thereby obtaining a cutting portion which has a better linearity but no burr. However, for a wire-electrode with a high tensile strength that has been frequently used in recent years as described above, even if it is cut by means of a mechanical stretching-cutting apparatus, the electrode wire will have been cut before it is sufficiently pulled and stretched since the elongation percentage is low, although the tnesile strength is high, thus the cutting edge portion does hot become straight. In addition, the electrode wire is cut so that the cross section of the cutting portion is about the same shape before cutting, and any burrs and the like are frequently formed on the cutting surface. Hence, it is difficult to pass the wire-electrode through the guides and nozzles of the working machine, the inserting hole of the workpiece or the like.
Therefore, there has been proposed to heat the wire electrode before applying the tensile force to the wire electrode in its axial direction, thereby melt-cutting or cutting while pulling and stretching the wire; this technique is disclosed in a U.S. Pat. No. 4,379,959 of 1981. However, in this well-known technique, since the control of the temperature and tensile force when cutting the wire electrode is insufficient, the shape of the cutting edge is unstable and there are such problems that the heating apparatus is large and the work is not rapidly done.